Surface structures of a Co-doped anatase TiO2 (001) film investigated by scanning tunneling microscopy

The surface structure of an anatase Ti0.94Co0.06O2 (001) film, grown epitaxially on a Nb-doped SrTiO3 (001) substrate, was investigated using in situ scanning tunneling microscopy. For the as-grown film, a (1×n) (n=3, 4, 5, and 6) reconstructed surface was observed that shows (n−2) faint rows between adjacent bright rows. After annealing at 650 °C, nanoparticles appeared, mostly on the step edges. From the I–V curves measured by scanning tunneling spectroscopy, the tunneling gap of Co:TiO2 was estimated to be about 3.0 eV, consistent with the band gap (Eg=3.2 eV) of pure anatase TiO2. However, on nanoparticles, the I–V curve showed a much smaller gap, suggesting that the particle must be different from TiO2.